With all due respect to all the fellow posters, however the only problem with all these recommendations is that the Acer is limited to a (poor) 220W PSU. Now with that power supply, I would be even supprised if it could run using anything over a GT 315 - well, yes it could but for how long.
Now, if you can upgrade your Power Supply from the 220W to at least a 400W, and I assume that your case can take a dual-slot card (the rear connector takes-up 2 or more pci-e slots), then you can get your hands on this video card from newegg:
Palit NE5S4500F0601 GeForce GTS 450.
Please note that this card requires a minimum 400 Watt power supply.
Do correct me if I am wrong, I just would not want anyone to fry their system due to unitended negligence. It probobably wont even start with the Palit card, but with a GT 430 or equivelant it might be a different and very short story.